the idea is that you have a variable int state that dictates the screen you are on:

When state is 0 you show the start screen.

If state is 1 you show the game (what you have in draw() now)

when state is 2 you show the game over screen.

In draw() you check what value state has and act accordingly. I use switch() to check this but if… else if… would be the same.

(you can even name the numbers 0,1,2 as constants)
(since a mousePressed means something different in the states (to which state you want to go next that is), you can use switch(state) in mousePressed too)